The assembly of pistons into internal combustion engines is a complex process. The piston rings must be compressed while the piston is aligned with the cylinder bore. Care must be taken not to damage either the rings, the cylinder walls or crankshaft split bearing of the connecting rod assembled to the piston. The bearing is comprised of two mating split journal sections, one of which is affixed to the end of the connecting rod. These bearing sections are carefully mated and machined together as a precautionary measure underscoring the importance of this assembly to the assembled engine. The assembly of the piston and bearing to the crankshaft must be accomplished very carefully so that the bearing surfaces are not marred and the alignment is proper.
Because of the many interrelated, critical steps which must be performed in mounting each piston in the engine, most automotive assembly lines still use assembly line manual labor for performing the many steps of the assembly.